Among the technologies for continuously casting molten metal, a technology that makes use of electromagnetic force during casting has been developed in order to attain stabilization of the bath level of the molten metal, a smooth surface of a continuously cast casting and an increase in casting speed.
For example, Japanese Unexamined Patent Publication No. S52-32824 discloses a method that is aimed at improving the surface appearance of a casting by, as shown in FIG. 11: supplying alternating current to an energizing coil 35 disposed so as to surround a mold 31 and insulated with refractories; curving the meniscus 33 of molten metal 32 and by so doing accelerating the inflow of powder 34; and reducing a contact pressure between the mold and the casting at the time of primary solidification. However, the problem of this method has been that induction current is induced in cooling copper plates composing a mold by an alternating magnetic field imposed by a magnet coil and, due to the surface effect, the magnetic field to be imposed on molten metal in the mold is attenuated.
Further, as a means of restraining the attenuation of a magnetic field in a mold for continuous casting and further enhancing the electromagnetic effect, Japanese Unexamined Patent Publication No. H05-15949 discloses a continuous caster for metal casting that is equipped with a metal mold 31 with an internal cooling construction and a magnet coil 35 that circles the mold and conducts a high frequency current, as shown in FIG. 10. In this continuous caster, the mold 31 has either (a), at the upper part of the mold, segments 37 with an internal cooling construction that are divided from each other by plural slits 36 which do not reach the top end of the mold and are parallel to the casting direction, or (b), at the upper part of the mold, segments 37 with an internal cooling construction that are divided from each other by plural slits 36 parallel to the casting direction and plural beams that connect the segments. A magnet coil 35 is disposed so as to circle the segments. Here, molten metal is supplied into the mold through an immersion nozzle 38.
However, with a mold having such slits, as the mold cannot be reinforced with back plates or the like, the rigidity of the mold is insufficient, the mold tends to be deformed thermally, and thus the mold has seldom been applied to a casting having a large section, such as a slab. In order to solve such problems, Japanese Unexamined Patent Publication No. 2000-246397 discloses a continuous caster for casting molten metal wherein an electromagnetic force is imposed in the direction perpendicular to the mold wall on the molten metal in the vicinity of a primarily solidified portion at a meniscus in a mold for continuous casting as shown in FIG. 9.
This mold 31 for continuous casting comprises: a magnet coil 35 that is disposed around the circumferential surface of the mold and conducts alternating current; a pair of the first sets each of which is composed of a first cooling copper plate 39 and a first back plate 41 made of nonmagnetic stainless steel; a pair of the second sets each of which is composed of a second cooling copper plate 40 and a second back plate 42 made of nonmagnetic stainless steel; and plural divided cooling parts including insulators 46. Each of the first cooling copper plates 39 and the second cooling copper plates 40 has at least one groove on the surface opposite the casting surface 49 and is airtightly fixed on the grooved surface to the relevant first back plate 41 or second back plate 42 with fastening bolts 44. Here, sealers 47 are inserted between each of the cooling copper plates and the relevant back plate. In this way, a coolant path 43 is formed by the groove(s) of each cooling copper plate and the relevant back plate.
Further, each of the first cooling copper plates 39 is electrically insulated from the adjacent second cooling copper plates 40 by the insulators 46 and also each of the first back plates 41 is fastened to the adjacent second back plates 42 with insulated fastening bolts 45 in an electrically insulated manner. This method has the advantages of being not only able to reduce the loss of electromagnetic force but also being able to secure machining accuracy and assembling accuracy by dividing the full circumferential length of a mold into units.
However, in the case of a mold which is configured so that the first cooling copper plates of the narrower side are interposed between the second cooling copper plates of the wider side and an insulator is placed at each of the mating faces 48 of each of the first cooling copper plates and the adjacent second cooling copper plates, when the first cooling copper plates slide and the width of a casting is changed, the insulator may be damaged and fall off due to the friction or the intrusion of foreign materials at the mating faces. Therefore, though it is possible to maintain the insulation at the early stage of casting, it is possible that, when the operations of width change and the like are repeated, insulation performance deteriorates and a desired electromagnetic force cannot be imposed.
In a commonly used width-variable mold, insulators made of Teflon (a registered trademark) or the like are sometimes interposed between copper plates to prevent scratches. However, the object is other than insulation and the copper plates partially touch each other electrically. Since the insulation resistance of the insulators is insufficient at these portions, a desired electromagnetic force is hardly imposed.